The object which has been long under water, e.g., a steamship, an oil pipe and an oil tank, to which an abnormal condition occurs, such as a collision, corrosion, underwater organisms and plant attachment, etc., needs local repairmen or replacement timely. Due to underwater work, there is no a simple, reliable and economical method to solve the problem so far. If the object is movable, it can be dragged on the water, e.g. the steamship can be dragged to a dock to be repaired. However, the cost is expensive, and the object is easy to damage during drag. While, if the object is unmovable, such as the oil pipe or the oil tank, there is not a simple, reliable and economical method to perform the operations such as replacement of part of the detachable member, local painting and the likes.